A variety of devices and methods have been employed in an effort to detect and/or monitor the progress of corrosion in well tubing strings, or pipes, and well casing strings, and the process is broadly referred to as "downhole" corrosion monitoring. As used herein, "corrosion" includes such defects as metal loss, pitting and cracking which, if left unchecked, can progress to result in a failure of the pipe.
Downhole corrosion monitoring is particularly important in the operation and management of oil gas or water wells and fields, not only in predicting the useful life of the well tubing and casings, for the purpose of avoiding failures during operation, but also in determining the efficacy of chemical additives intended to minimize such corrosion.
Although the methods presently employed for monitoring downhole corrosion vary, they all require the use of wire lines to install and/or retrieve devices placed at predetermined positions or the running of logging tools. These prior art methods include wireline logging tools that are attached to the end of a wire or cable; coupons set and recovered by wireline; and programmable electronic probes set and retrieved by wireline. In order to use any of these methods, the well has to be taken out of service. Shutting down the well on a regular schedule for corrosion monitoring is costly, not only in terms of direct labor charges, but also in terms of lost production and revenues. Additionally, disruption of the flow due to the installation of intrusive devices in the wellbore can give rise to misleading corrosion rate data.
Apparatus and methods utilizing ultrasound to measure piping wall thickness and to detect defects are known for installed well tubing and casing, but must be run by wireline and suffer the same limitations as all such intrusive tools. Also, because of the imprecise positioning of the wireline tools from one inspection to the next, it is not possible to obtain reliable data on the in situ rate of corrosion. Another major limitation of existing ultrasonic wireline devices is the requirement that they need to be run in a liquid-filled tube in order to transmit data. This requirement limits their use in multi-phase and gas wells.
It is therefore an object of this invention to provide an apparatus and method that will permit downhole corrosion monitoring without taking the well out of service or disrupting the flow, and that can be used in all types of well service, i.e., water, oil, gas and/or multi-phase wells.
It is another object of the invention to permit corrosion monitoring data to be obtained and analyzed with any desired frequency, or even continuously.
Another object of the invention is to permit corrosion monitoring data to be obtained from the time of the installation of well tubing and/or well casing strings to provide a baseline, and thereby to identify the onset of corrosion as well as its rate of progress in the section or sections of tubing being monitored.
It is also an object of the invention to provide an economical and cost-effective method and apparatus for in situ downhole corrosion monitoring that will provide reliable data without resort to wirelines and intrusive tools and methods.